securing pop3 sessions

[spacetask]

I'm using qmail 1.0.3 with the included qmail-pop3d.

What's the best way on the server side to prevent passwords from being
sent as clear text over the network for a pop3 session?  I know users
will be reluctant to change their mua's. So what can I do on my side?
Is there any way around this without expecting anything from the pop
users?